The present invention relates to polypeptide antigens suitable for providing protective immunity against malaria by incorporation into a vaccine. These antigens have amino acid sequences corresponding to segments of the amino acid sequence of the circumsporozoite protein that lie outside the bounds of the tandemly repeated domain of such protein. The antigens of the present invention can be used to elicit formation of antibodies, which recognize sporozoites not only of the same species of plasmodium from which these antigens were derived, but of other species as well.
The present application incorporates by reference the entire disclosures of:
(a) U.S. Pat. No. 4,466,917 of Nussenzweig, R., et al., issued on Aug. 21, 1984;
(b) assignee's copending U.S. patent application Ser. No. 574,553 of Ellis, J. et al., filed on Jan. 27, 1984 and entitled Protective Peptide Antigen; and
(c) assignee's copending U.S. patent application Ser. No. 633,147 of Ellis, J. et al., filed on Jul. 23, 1984 and entitled Protective Peptide Antigen Corresponding to Plasmodium Falciparum Circumsporozoite Protein. Pertinent excerpts of the disclosures of these applications have been published in U.S. Pat. No. 4,915,942 of which the present is a continuation.
In most instances, malaria infections are initiated by the introduction of sporozoites (highly immunogenic forms of the malaria parasite) into the bloodstream of a host through the bite of an infected mosquito. The immunogenicity of sporozoites resides largely, if not exclusively, in a single antigen, the circumsporozoite (CS) protein (described in detail by F. Zavala, A. H. Cochrane, E. H. Nardin, R. S. Nussenzweig and V. Nussenzweig in an article in J. Exp. Med. 157:1947 (1983). G. N. Godson, et al., Nature 305:29 (1983) reported that the immunogenicity of the CS protein is restricted almost entirely to a singe epitope which is identically or quasiidentically repeated several times in tandem. See also V. Enea, et al. Proc. Nat'l Acad. Sci. (accepted for publication, 1984).
Circumsporozoite proteins (CS proteins) are members of a family of polypeptides comprising the surface membranes of mosquito salivary gland sporozoites of mammalian malaria parasites of the genus plasmodium. The strong immunogenic properties of sporozoites are associated mainly with the CS protein. This protein, specific for the sporozoite stage, has an immunodominant region of repetitive epitopes. The repeated sequence from N to C terminus of the CS protein for P. knowlesi is Gln-Ala-Gln-Gly-Asp-Gly-Ala-Asn-Gly-Gln-Pro-(also designated as QAQGDGANGQP) and the repeated tetrapeptide sequence of the CS protein for P. falciparum is Asn-Ala-Asn-Pro-(also designated as NANP). Synthetic peptides consisting of multiples or analogs of the repeated amino acid sequences have been shown to be antigenic and are useful in the development of a malaria vaccine. Unfortunately, however, peptides derived from the immunodominant region of the CS protein display very little homology among themselves and only species-specific antigenicity.
Due to the immunodominance of the repetitive epitopes of the CS protein, it had not heretofore been possible to determine if other segments of the CS protein, which are not within the repetitive domain sequence, can induce antibodies affecting the viability of the parasite. Clearly, immunogenicity itself does not establish the utility of peptides having the sequence of such non-repeating segments as protective antigens against sporozoites, since such segments would need to be on an exposed surface of the CS molecule to allow recognition by antibodies.